With rapid spread of mobile terminals and wireless communications, an RF (Radio-frequency) device primarily intended for a communication interface has attracted attention and it is considered to rapidly evolve in the future. In recent mobile terminals and personal computers, devices with low power consumption have been increasingly adopted. In such cases, using an SOI substrate provides a merit of a reduction in power consumption due to a decrease in leak current. Further, using SoC (System on a Chip) or the like to incorporate the RF device is also considered, and using an SOI wafer can provide a merit of reducing crosstalk between devices in terms of characteristics of an RF transistor.
Here, the crosstalk means propagation of an undesired electrical signal between devices, and the signal is propagated through a capacitor between wiring lines of devices or through a wafer, for example. Although the crosstalk is reduced as a resistivity of the wafer is increased, a wafer resistivity of a device forming portion cannot be actually extremely increased.
However, when the SOI wafer is used, since a buried oxide film layer (a BOX layer) is present between the SOI layer and the base wafer, the crosstalk can be reduced. Furthermore, since a device is not formed on the base wafer serving as an underlying layer of the BOX layer, a high-resistance base wafer can be used without being subject to limitation of device formation, thereby improving RF characteristics.
However, when the high-resistance base wafer is used, electric field can be applied to the BOX layer depending on an arrangement of sources/drains and other electron circuits produced on the SOI layer to generate a inversion layer on an interface between the BOX layer and the base wafer. In such a case, a desired radio-frequency characteristic cannot be obtained in spite of using a high-resistance wafer, since the radio-frequency characteristic is adversely affected. It is to be noted that as the high-resistance, a substrate of 1000 Ω·cm or more is generally used in many cases.
As a countermeasure, it is considered that the degradation of radio-frequency characteristics due to the inversion layer can be prevented by increasing an interface state density (hereinafter, also referred to as Dit) to trap carriers in an interface state.
As such a technology, for example, it has been, known a technology that introduces an intermediate layer (a trap layer) such as a polysilicon layer or a nitride oxide to an interface between a BOX layer and a base wafer, and can obtain an SOI wafer with good radio-frequency characteristics thereby so as not to form a inversion layer (see, e.g., Patent Documents 1 and 2, Non-Patent Document 1).
It has been also introduced a method in which a silicon wafer having the plane orientation different from the silicon wafer to be an SOI layer is used as a base wafer to prevent deterioration of radio-frequency characteristics (e.g., Patent Document 3).
The foregoing technologies are essential to obtain good radio-frequency characteristics. In evaluating the technologies, substrate resistance can be measured by a method such as a spreading resistance measurement and so on, for example. However, there is not an evaluation method to directly measure characteristics of a trap layer (a functional layer) immediately under a BOX layer. Accordingly, there has been a problem that the radio-frequency characteristics can be evaluated only by a method in which an actual device is produced to measure a leakage power, etc.